With the mounting or wiring of semiconductor chips by means of automatic assembly machines, known in the trade as Die Bonders or Wire Bonders, such a clamping element is suitable for the transport of substrates in a predetermined direction. A transport apparatus working with clamping elements of this kind is known from U.S. Pat. No. 5,163,222, while such a clamping element is known from CH 679,878.
Many different materials are used as substrates. On the one hand, metal strips punched out of a sheet, so-called leadframes, serve as substrates. As a rule, such leadframes are still comparatively thick and quite stable. On the other hand, foil strips made of plastic are used as substrates which are as thin as a piece of paper. The substrates are taken from a stack or a magazine and fed to a transport apparatus containing at least one clamping element by means of which they are moved in cycles in the transport direction and fed to at least one processing station. Generally, a Die Bonder comprises two processing stations, namely a dispensing station, where a portion of adhesive is applied to a substrate location, and a bonding station, where a semiconductor chip is placed onto the substrate. On the other hand, a Wire Bonder comprises only one single processing station where the semiconductor chips are wired to the substrate. After processing, the substrates are transported further and inserted into a magazine. Such a magazine is a container with numerous grooves arranged one above the other and the substrates are inserted into the spaces between them. Unfortunately, it often happens that a substrate catches or jams on some part of the magazine before it has reached its final position in the magazine. In doing so, the substrate can be damaged or even destroyed especially when it is relatively thin.
In order to prevent damage to or destruction of the substrate in such a case, with some automatic assembly machines a roller is used which rests on the edge of the substrate. When the substrate is transported in transport direction, the roller rolls on the substrate and delivers a signal that it is rotating. As soon as the substrate catches somewhere, then its position no longer changes in relation to the roller: The roller is stationary. The roller standstill initiates a signal in order to immediately stop the transport apparatus. This detection method functions sufficiently well with relatively stable substrates however not with thin foil strips made of plastic.
Another method practised consists in reducing the clamping force produced by the clamping element for holding the substrate as far as possible so that the clamping element simply slides along the substrate as soon as the substrate catches somewhere. On the one hand, setting the clamping element is a difficult task. On the other hand, a special drive is necessary for the clamping element so that the clamping jaws open and close reliably. Also, this method can no longer be used with thin foil strips.